The Lamplighters League

This pops after completing the story mission in week 4. Take out the first group of enemies, then approach the yellow marker to activate the artifact. Three fire zombies will pop out though. They can revive even if you defeat them so just run away to the escape point on your turn. There are normal enemies patrolling near the exit area so take down only those you need to while progressing to the exit. The fire zombies will continue following you so keep moving forward to avoid getting attacked from them at your back.